FIG. 8 is a perspective view of a conventional motor-driven slide type variable resistor, and FIG. 9 is a front cross sectional view of a follower-pulley mounter, an essential portion of the resistor. A frame 1 is made of a long, L-shaped metal strip having a linear slit 1B provided lengthwise in the center region of a planar side 1C thereof. A slide type variable resistor unit 2 is joined by screws 1A to the lower side of the frame 1, so that an operating lever 7 extends upward through the slit 1B of the frame 1. With a rotary shaft 3A being arranged vertically, a motor 3 is fixedly mounted to one lengthwise end of the frame 1 by motor retaining screws 3B which extend through corresponding screw slots 3C. A toothed pulley 4 is mounted to the upper end of the rotary shaft 3A. The frame 1 has a columnar pin 10 anchored by a retainer screw 11 to the other lengthwise end thereof, and a follower pulley 5 is rotatably mounted to the pin 10. A toothed belt 6 of an annular shape is mounted under tension between the toothed pulley 4 and the follower pulley 5. The operating lever 7 of the variable resistor unit 2 is fixedly joined by a resin joint 8 to an intermediate portion of the belt 6.
The slide type variable resistor unit 2 can operate manually. As shown in FIG. 9, a slider 13 is supported at the bottom with a resilient contact 12 and is accommodated for linear movement along the lengthwise direction in a long, box-like housing which includes a long bottom-opened metal case 14 and a resistor circuit board 15 mounted to the lower side of the metal case 14 to close the bottom opening of the metal case 14.
The slider 13 may operate with the motor 3 driving the toothed belt 6 and with a hand holding a knob 9 mounted to the upper end of the operating lever 7.
For example, plural conventional motor-driven slide type variable resistors may be mounted in a row in an electronic apparatus, such as an audio mixer. If respective operating levers 7 driven by the motors 3 in the resistors travel at different speeds from each other, the variable resistor units 2 may be declined in the performance at combination. Therefore, the operating levers 7 preferably travel at identical speeds.
A variation of tensioning of the toothed belts 6 resulting from variations in their size and a change in the number of revolutions of the motor 3 can be compensated by the following procedure. The motor retaining screws 3B are loosened, and the motor 3 is then re-positioned along the screw slots 3C in the frame 1 to tension the toothed belt 6 at an optimum. Then, the motor 3 is tightened again. This, however, consume a considerable length of time.